The present invention relates to a process for the preparation of a carboxylic acid. Particularly, the present invention relates to a process for the preparation of a carboxylic acid by the reaction of an alcohol with carbon monoxide in the presence of a palladium compound as catalyst, an organic or an inorganic halide and an organic sulphonic acid as promoter, water and a solvent under mild conditions.
Acetic acid has been produced industrially on a large scale by methanol carbonylation by the well known Monsanto and Cativa(trademark) processes. U.S. Pat. No. 3,816,490, European Patents EP 728726A1 and EP 752406A1 disclose the use of rhodium or iridium as catalyst to produce acetic acid using methanol and carbon monoxide in the presence of iodide promoters and water. The disadvantage of these processes is the high costs and limited availability of the catalyst. The use of nickel in the presence of iodides has been reported for the carbonylation of methanol to acetic acid [U.S. Pat. No. 4,902,569 and J. Catal. (156), 290, (1995)]. However these processes use drastic conditions.
Palladium and platinum catalysts carbonylation of methyl iodide to acetic acid with quaternary iodide promoter is disclosed in J. Chem. Soc. Chem. Commun. 179-180 (1999). However the activity level is low (Turn Over Number TON=110), and methyl iodide is used as starting material. European Patent EP 0133331 discloses the palladium catalysed carbonylation of methanol to acetic acid in the presence of metal iodide, methyl iodide and sulfolane or sulfoxide. However, in this process sulfolane or sulfoxide are essential components.
As can be seen, the prior art processes suffer from several disadvantages such as the use of expensive starting materials resulting in higher cost of production; low activity; or require extreme operating conditions thereby rendering the process expensive. It is therefore important to develop a process for the preparation of a carboxylic acid that overcomes the drawbacks enumerated above.
The main object of the present invention is to provide a process for the preparation of a carboxylic acid that overcomes the twin drawbacks of low activity and drastic reaction conditions.
It is another object of the invention to provide a process for the preparation of a carboxylic acid that operates at milder reaction conditions.
It is a further object of the invention to provide a process for the preparation of a carboxylic acid such as acetic acid using metal halides and organic sulphonic acid as promoters.
It is yet another object of the invention to provide a process for the preparation of a carboxylic acid that is efficient and less expensive.
These and other objects of the invention are achieved by providing a palladium catalysed process for the carbonylation of an alcohol to the corresponding carboxylic acid in the presence of halide and organic sulphonic acid promoters, water and a solvent at milder reaction conditions.
Accordingly, the present invention provides a process for the preparation of a carboxylic acid comprising carbonylating the corresponding alcohol in carbon monoxide atmosphere and in the presence of water, a solvent, a palladium catalyst and a promoter system consisting of an organic or inorganic halide and an organic sulphonic acid, at a temperature in the range of 50-250xc2x0 C., at a pressure in the range of 50-2000 psig for 1 to 10 hours, the concentration of the catalyst being one mole of catalyst per every 50-50000 moles of the alcohol, the amount of the organic or inorganic halide being in the range of 5-500 moles per mole of the catalyst, and the amount of the organic sulphonic acid being in the range of 5-500 moles per mole of the catalyst, collecting the resulting product.
In one embodiment of the invention, the alcohol used is ROH wherein R is an alkyl group having 1 to 5 carbon atoms.
In another embodiment of the invention, the catalyst used comprises palladium (II) or palladium (0) compound selected from the group consisting of palladium chloride, palladium bromide, palladium iodide and palladium acetate; or a metal complex of palladium selected from the group consisting of bis(triphenylphosphino)dichloropalladium (II), bis(triphenylphosphino)bibromopalladium (II), bis(triparatolylphosphino)dichloropalladium (II), bis(triparatolylphosphino)bibromopalladium (II), tetrakis(triphenylphosphino) dichloropalladium (0) and tetrakis(triphenylphosphino)dibromopalladium (0).
In a further embodiment of the invention, the organic halide used is of the formula RX wherein R is an alkyl group having 1 to 5 carbon atoms and X is a halogen selected from Cl, Br, and I.
In another embodiment of the invention, the inorganic halide used is of the formula MX wherein M is hydrogen or an alkali metal selected from the group consisting of Li, Na and K, and X is a halogen selected from Cl, Br and I.
In another embodiment of the invention, the organic sulphonic acid used is selected from the group consisting of benzene sulphonic acid, paratolyl sulphonic acid, methane sulphonic acid, and trifluoromethane sulphonic acid.
In a further embodiment of the invention, the solvent used comprises a ketone selected from the group consisting of acetone, diethyl ketone and acetophenone; or a cyclic ether selected from tetrahydrofuran and dioxan; or a nitrile selected from acetonitrile and benzonitrile; or an organic solvent selected from toluene and benzene.
In another embodiment of the invention, the concentration of the catalyst is one mole of catalyst for every 100-5000 moles of the alcohol, preferably one mole of catalyst for every 150-1000 moles of the alcohol.
In another embodiment of the invention, the amount of organic or inorganic halide promoter used is in the range of 25 to 150 moles per mole of catalyst.
In yet another embodiment of the invention, the amount of the organic sulphonic acid used is in the range of 50-150 moles per mole of catalyst.
In a further embodiment of the invention, the amount of water is in the range of 1 to 30%(v/v) of the total reaction mixture, preferably 3-25%.
In another embodiment of the invention, the pressure of the carbon monoxide used is in the range of 200-1000 psig.
In a further embodiment of the invention, the temperature of the reaction mixture is in the range of 70-200xc2x0 C.